1. Field of the Invention
In the completion of oil and gas wells, there are various downhole operations in which it may become necessary to isolate particular zones within the well. This is typically accomplished by temporarily plugging off the well casing at a given point or points with a bridge plug. Bridge plugs are particularly useful in accomplishing operations such as isolating perforations in one portion of a well from perforations in another portion, or for isolating the bottom of a well from a wellhead. The purpose of the plug is simply to isolate some portion of the well from another portion of the well. However, in some instances, the bridge plug may not necessarily be used for isolation, but may be used, for example, to create a cement plug in the wellbore. The bridge plug may be temporary or permanent; if temporary, it must be removable.
Bridge plugs may be drillable or retrievable. Drillable bridge plugs are typically constructed of a brittle metal such as cast iron that can be drilled out. One typical problem with conventional drillable bridge plugs, however, is that without some sort of locking mechanism, the bridge plug components may tend to rotate with the drill bit, which can result in extremely long drill-out times, excessive casing wear, or both. Long drill-out times are highly undesirable, as rig time is typically charged by the hour.
An alternative to drillable bridge plugs is the retrievable bridge plug, which may be used to temporarily isolate portions of the well before being removed, intact, from the well interior. Retrievable bridge plugs typically have anchor and sealing elements that engage and secure it to the casing wall. To retrieve the plug, a retrieving tool is lowered into the casing to engage a retrieving latch, which, through a retrieving mechanism, retracts the anchor and sealing elements, allowing the bridge plug to be pulled out of the wellbore. A common problem with retrievable bridge plugs is the accumulation of debris on the top of the plug, which may make it difficult or impossible to engage the retrieving latch to remove the plug. Such debris accumulation may also adversely affect the relative movement of various parts within the bridge plug. Furthermore, with current retrieving tools, jarring motions or friction against the well casing can cause accidental unlatching of the retrieving tool, or re-locking of the bridge plug (due to activation of the plug anchor elements). It may also be difficult to separate the retrieving tool from the plug upon removal, necessitating the use of additional machinery. Problems such as these sometimes make it necessary to drill out a bridge plug that was intended to be retrievable.
Thus, there is a need in the art for a bridge plug whose performance is not impaired by undesirable conditions such as differential pressure zones or wellbore debris, and that may be removed from the wellbore without undue exertion or cost.